Gases for analysis may contain large and small particulate matter, condensates, and other contaminants as well as the constituents to be analyzed. For example, the exhaust gas of internal combustion engines is a highly complex mixture which must be filtered in order to allow analysis of constituents. Because of the wide variety of contaminants, several different types of filters are typically required to properly prepare a gas for analysis. Adding to the difficulties, contaminants such as particulate matter and condensates can harm filters, pumps, and the analyzer as well as cause operational inefficiencies. These inefficiencies include clogged filters, pump fouling, and void volumes resulting in high pump flow rate requirements.
Engine exhaust gases typically contain coarse and fine particulate matter, both liquid and vapor water, other condensates, and hydrocarbons, carbon monoxide, carbon dioxide, nitrogen oxides, oxygen, and other compounds to be analyzed. Contaminants must be filtered out by different type filters suited for the particular contaminant in order to accurately measure the constituents of the gas. The general filtration process involves first removing large particles, liquid water, and other condensates using a relatively coarse filter. The condensate is filtered by another finer filter before being drawn off by a pump. The filtered gas is then filtered for finer particles (typically less than 5 micron particles) by another filter assembly. In particular, the removal of all water traces before the final filter is important because water absorbs the hydrocarbons which is one of the gases to be analyzed. If water is present in the final fine filter, the hydrocarbon measurements will be inaccurate. Further, certain materials can absorb molecules to be analyzed. These absorption phenomena are termed "hydrocarbon hang-up" or "molecular hang-up." Thus, the filtration process is extremely important for accurate measurements of gas constituents and to protect the analyzer and pumps.
Typical prior art filtration devices utilized separate filter assemblies for each step in the filtration process. This required additional labor and time-consuming changes of devices. If the devices were connected, complicated interconnections of each unit would typically produce large void volumes. The void volumes necessitated high pump flow rates to move the gas through the filters and further significantly increased response times for the filter assembly in operation with an analyzer. The high pump rates also produce increased stress on system components.
Knight et al. U.S. Pat. No. 3,527,027 discloses a pressurized gas co-axial multifilter apparatus having cylindrical coarse and fine filters disposed concentrically about a common axis. At the bottom of the housing is a water bowl for trapping water. While Knight et al. performs the filtration functions in a single unit, it is not suitable for exhaust gases containing relatively large amounts of water. Exhaust gas condensate in a pressurized gas would soak the filters, rendering them inefficient and subject to hydrocarbon absorption. Because of Knight et al.'s co-axial configuration, the surface areas of the different filters are limited in size to that which fits concentrically in the apparatus. In particular for the final fine filter for exhaust gas analysis, it is preferable to use larger filters which are more efficient and last longer. If the surface area of Knight et al.'s fine filter is increased, a large void volume would be produced. This would then necessitate a high pump flow rate to move the gas through the system which would require more powerful pumps and greater costs. Because of the different substances and amounts to be filtered, the usable lifetimes of different filters are significantly different. Thus in maintaining the filters, it would be desirable to be able to remove each filter separately. To remove filters for replacement or cleaning in Knight et al.'s device requires opening the housing and removing of the entire internal assembly. That is, the removal of specific filters would be difficult because of their proximity and concentricity. Finally, exhaust gas analyzers typically operate by suction of exhaust gas into the analyzer rather than from compressed air injection as required in Knight et al.'s device.